Referring to FIG. 1A, a block diagram of a fan speed controller for controlling the operating speed of a fan 190, according to the prior art, is shown. The fan 190 is typically utilized to cool electronic devices that include, but are not limited to, computers, game consoles, microprocessors, graphics processor and the like. The cooling circuit includes a pulse width modulation (PWM) generator 110, a drive transistor 145 and a fan 190. The speed of the fan 190 is controlled by a PWM signal (VPWM) generated by the PWM generator 110.
An exemplary PWM generator 110 includes a reference voltage generator 115, an operational amplifier 120, a comparator 125 and a signal generator 130. The operational amplifier 120 receives a feedback signal (VFB) (e.g., fan speed signal, temperature signal or the like) and the reference voltage (VREF) generated by the reference voltage generator 115. The operational amplifier 120 generates a control signal (VCONTROL) having a voltage that is a function of the difference between the feedback signal (VFB) and the reference voltage (VREF). The signal generator 130 generates a signal having a repetitive wave form (VRWF) (e.g., sawtooth or the like). The comparator 125 receives the control signal (VCONTROL) and the repetitive wave form signal (VRWF). As depicted in FIG. 1B, the comparator 125 generates the PWM signal (VPWM) (e.g., a periodic rectangular wave) as a function of the relative difference between the control signal (VCONTROL) and the repetitive wave form signal (VRWF). The output of the comparator 125 is at a first state (e.g., “on”) when the repetitive wave form signal (VRWF) is less than the control signal (VCONTROL). The output of the comparator 125 is at a second state (e.g., “off”) when the repetitive wave form signal (VRWF) is greater than the control signal (VCONTROL). Thus, the duty cycle of the PWM signal (VPWM) is a function of the feedback signal (VFB).
The switching speed of the PWM signal is typically a few tens of Hertz (Hz), which is in the audible frequency range (e.g., 20-20,000 Hz). The PWM signal (VPWM) controls the switching of the drive transistor 145. The drive transistor 145 is on during the duty cycle of the PWM signal (VPWM). Accordingly, a drive current flows through the fan 190. The longer the duty cycle in a given period, the longer the fan 190 is driven. The operating speed of the fan 190 increases the longer the fan 190 is driven.
Referring now to FIG. 1C, timing diagrams of exemplary PWM signals (VPWM) are shown for controlling fan speed. The PWM signals (VPWM) are composed of rectangular pulse trains having a constant period (T). The duration of the duty cycle (ton) is varied to control the operating speed of the fan 190. The operating speed of the fan 190 is a function of the duty cycle of the PWM signal (VPWM) while the frequency remains constant. For example, the fan 190 is operated at a slow speed by applying a PWM signal (VPWM) having a relatively short duty cycle 192. The fan 190 is operated at a fast speed by applying a PWM signal (VPWM) having a relatively long duty cycle 196.
Direct PWM fan speed control is advantageous because relatively high efficiencies can be achieved, e.g., energy supplied to the fan divided by the input energy can be 85% or more. However, direct PWM fan speed control is problematic in that the fan 190 may emit audible noise as a result of the time varying drive current through the fan 190.
To avoid the audible noise, direct current (DC) regulation may be utilized to drive the fan. Referring now to FIG. 2A, a block diagram of another fan speed controller 205, according to the prior art, is shown. The linear voltage fan speed controller 205 includes an analog voltage generator 270 and an amplifier 275. The analog generator 270 generates a linear voltage having a varying voltage level. The linear voltage is amplified by an amplifier 275 before driving the fan 290. It is appreciated that the amplifier 275 may be integral to the analog voltage generator 270. As depicted in FIG. 2B, the speed of the fan 290 is a function of the voltage level of the linear voltage generated by an analog voltage generator 270. For example, the fan 290 is operated at a slow speed by applying a linear voltage having a relatively low voltage level 292. The fan 290 is operated at a fast speed by applying a linear voltage having a relatively high voltage level 294.
Driving the fan 290 with a linear voltage reduces the fan noise. However, amplification of the linear voltage results in relatively high losses as compared to direct PWM fan speed controllers. The efficiency of a linear controlled fan is defined as output voltage over input voltage (VIN/VOUT). Thus, the slower the fan (e.g., lower output voltage) the lower the efficiency, because the input voltage is constant. Usual regulation ranges can result in efficiencies of about 50% to 75%.
Referring now to FIG. 3, a block diagram of another fan speed controller 305, according to the prior art, is shown. The fan speed controller 305 includes a PWM generator 310, a low pass filter (e.g., resistor-capacitor (RC) circuit) 320 and an amplifier 330. The speed of the fan 340 is controlled by low pass filtering a PWM signal to generate a linear voltage. The voltage level of the linear signal is a function of the duty cycle of the PWM signal. For example a relatively low voltage level is generated by RC low pass filtering a PWM signal having a relatively short duty cycle. A relatively high voltage level is generated by RC low pass filtering a PWM signal having a relatively long duty cycle.
Driving the fan 390 with a linear voltage reduces the fan noise. However, amplification of the linear voltage results in relatively high losses as compared to direct PWM fan speed controllers. The efficiency of a linear controlled fan is defined as output voltage over input voltage (VIN/VOUT). Thus, the slower the fan 340 (e.g., lower output voltage) the lower the efficiency, because the input voltage is constant. Usual regulation ranges can result in efficiencies of about 50% to 75%.